Electrical discharge machining equipment

ABSTRACT

An electrical discharge machining apparatus compensates for an inductance fall component and a resistance fall component in a sensed machining gap voltage. The inductance fall component is determined by a current variation rate sensed by an instrument current transformer. The resistance fall component is sensed by a resistor, an integrator, or a combination of a second instrument current transformer and an ingegrator. The sensed gap voltage is then compensated for the resistance fall component and the inductance fall component.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improvement applicable to electricaldischarge machining equipment. More specifically, this invention relatesto an improvement applicable to means for sensing the voltage across thegap in which electrical discharges occur for electrical dischargemachining.

2. Description of the Related Art

It is required to maintain the geometrical length of the gap acrosswhich electrical discharges occur for electrical discharge machining ata constant value, for the purpose of maintaining a stable intensity foran electric current flowing in the gap for electrical dischargemachining. To satisfy this requirement, it is essential to employ meansfor sensing the voltage across the gap in which electrical dischargesoccur for electrical discharge machining (Hereinafter referred to as theelectric discharge machining gap voltage sensing means).

Presented below will be a description of electrical discharge machiningequipment described paying particular attention to electric dischargemachining gap voltage sensing means.

Referring to FIG. 1, a machining electrode 1 can be a rod electrode(which is employed in electrical discharge machining equipment forproducing a recess), as shown in the drawing, or a wire electrode (notshown but employable in wire electrode type electrical dischargemachining). A gap 12 in which electric discharges occur (hereinafterreferred to as a machining gap) is maintained between the machiningelectrode 1 and a workpiece 2 to allow electric discharges to occurtherein. Means 3 for generating a high frequency pulse wave electricvoltage is allowed to select variable wave forms. An instrument currenttransformer 4 or the like is designed to sense the variation rate ofelectric current. In the equipment illustrated in FIG. 1, thetransformer 4 is actually employed as a means for sensing an inductancefall component. An electric discharge machining gap voltage is sensed ata point 5, at which point the circuit of the high frequency pulse waveelectric voltage generating means 3 is connected to the rod machiningelectrode 1, in the case of electrical discharge machining for producinga recess in the workpiece 2. On the other hand, an electric dischargemachining gap voltage is sensed by means of electric feeder means or thelike (not shown), in the case of wire electrode type electricaldischarge machining, in which a wire electrode is employed.

An electric discharge machining gap voltage sensed at any of the pointsdescribed above contains not only the genuine value of the electricdischarge machining gap voltage but also error voltage components. Theerror voltage components include an inductance fall component and aresistance fall component which occur between the machining electrode 1and the electric discharge machining gap voltage sensng point 5. Theerror voltage components also include an inductance fall component and aresistance fall component which occur between the workpiece 2 and theother electric discharge machining gap voltage sensing point (which isnot shown in the drawing, but is one terminal of the instrument currenttransformer 4 or the ground point in this example). Particularly in thecase of wire electrode type electrical discharge machining equipment,the inductance fall component is so large that the inductance fallcomponent cannot be ignored. Therefore, it is required to sense theerror voltage components and to subtract the voltage amount from thevoltage between the two points at which an electric discharge machininggap voltage is sensed (between one end of the machining electrode 1 andthe ground point in this embodiment), for the purpose of correcting theapparent electric discharge machining gap voltage sensed in the mannerdescribed above and to obtain the correct or true electric dischargemachining gap voltage which is actually applied across the electricaldischarge machining gap.

Means for removing the influence of the aforementioned error voltagecomponents can be realized by employing a principle that the inductancefall component accounting for the major portion of the error voltagecomponents is proportional to the variation rate of an electric current.Therefore, electrical discharge machining equipment available in theprior art is provided which electric discharge machining gap voltagecorrecting means 6 which functions to subtract the inductance fallcomponent, which is determined following the output signal of theelectric current variation rate sensing means 4 such as an instrumentcurrent transformer or the like, from the apparent electric dischargemachining gap voltage (the voltage difference between the potential atone end of the machining electrode 1 and the ground potential, in thisexample) sensed in the aforementioned manner. The electric dischargemachining gap voltage correcting means 6 is inherently an adder. Theapparent electric discharge machining gap voltage (the voltagedifference between the potential at one end of the machining electrode 1and the ground potential, in this example) sensed in the aforementionedmanner is applied to a first element 61. The output signal of theelectric current variation rate sensing means 4 is applied to a secondelement 62 of the correcting means 6. Since the output signal of theelectric current variation rate sensing means 4 is a negative value, theelectric discharge machining gap voltage correcting means 6 actuallyperforms subtraction.

Adjustment of the second element 62 is readily possible by adjusting theoutput signal of the electric discharge machining gap voltage correctingmeans 6 to 0 (zero). This is accomplished by short-circuiting themachining gap 12 and applying a high frequency pulse voltage between thepoint 5 and one end of the instrument current transformer 4.

Although the electric discharge machining gap voltage correcting means 6is remarkably effective to remove the harmful effects of the inductancefall component, the harmful effects of the resistance fall component cannot be ignored, particularly in the case where a thin wire electrode ora tungsten wire electrode is employed.

SUMMARY OF THE INVENTION

For such cases where the harmful effects of the resistance fallcomponent can not be ignored, there is a need for more accurate electricdischarge machining gap voltage sensing means which are further providedwith means for correcting the error caused by the resistance fallcomponent.

The object of this invention is to satisfy the aforementionedrequirement and to provide electrical discharge machining equipmentprovided with more accurate electric discharge machining gap voltagesensing means, the voltage sensing means being further provided withmeans for removing the error caused by the resistance fall component.

An electrical discharge machining apparatus for machining a workpieceprovided with electric discharge machining gap voltage sensing means inaccordance with the first embodiment of this invention is an electricaldischarge machining apparatus provided with a machining electrode (1), ahigh frequency pulse voltage generating means (3) for supplying highfrequency voltage pulses across a machining gap (12) remaining betweenthe machining electrode (1) and the workpiece (2) to be machined, ameans (4) for sensing the variation rate of the electric current whichflows through the machining gap (12), and a means (6) for correcting theelectric discharge machining gap voltage by subtracting the inductancefall component which is determined following the electric currentvariation rate sensed by means of the electric current variation ratesensing means (4), from the electric discharge maching gap voltagerepresenting the voltage difference sensed across the machining gap(12). The electrical discharge machining equipment is further providedwith a machining current resistance fall component sensing means (7) forsensing the intensity of the electric current flowing in the machininggap (12), and the electric discharge machining gap voltage correctingmeans (6) being further provided an element (63) for subtracting themachining current resistance fall component sensed by the machiningcurrent resistance fall component sensing means (7), from the electricdischarge machining gap voltage representing the voltage differencesensed across the machining gap (12).

An electrical discharge machining apparatus provided with an electricdischarge machining gap voltage sensing means in accordance with thesecond embodiment of this invention is an electrical discharge machiningapparatus provided with a machining electrode (1), a high frequencypulse voltage generating means (3) for supplying high frequency voltagepulses across a machining gap (12) which remains between the machiningelectrode (1) and a workpiece (2) to be machined, a means (4) forsensing the variation rate of the electric current which flows throughthe machining gap (12), and a means (6) for correcting the electricdischarge machining gap voltage by subtracting an inductance fallcomponent which is determined following the electric current variationrate sensed by means of the electric current variation rate sensingmeans (4), from the electric discharge machining gap voltagerepresenting the voltage difference sensed across the machining gap(12). The electric current variation rate sensing means (4) is furtherprovided with an integrator (8) for integrating the electric currentvariation rate sensed by the electric current variation rate sensingmeans (4) to determine a machining current fall component, and theelectric discharge machining gap voltage correcting means (6) is furtherprovided with an element (63) for subtracting the machining currentresistance fall component determined by the integrator (8).

An electrical discharge machining apparatus provided with an electricdischarge machining gap voltage sensing means in accordance with thethird embodiment of this invention is an electrical discharge machiningapparatus provided with an machining electrode (1), a high frequencypulse voltage generating means (3) for supplying high frequency voltagepulses across the machining gap (12), which remains between themachining electrode (1) and a workpiece (2) to be machined, a means (4)for sensing the variation rate of the electric current which flowsthrough the machining gap (12), and a means (6) for correcting theelectric discharge machining gap voltage by subtracting the inductancefall component, which is determined following the electric currentvariation rate sensed by means of the electric current variation ratesensing means (4), from the electric discharge machining gap voltagerepresenting the voltage difference sensed across the machining gap(12). The electric current variation rate sensing means (4) is furtherprovided with a second electric current variation rate sensing means (9)and an integrator (8) for integrating the electric current variationrate sensed by the second electric current variation rate sensing means(9), and the electric discharge machining gap voltage correcting means(6) is further provided an element (63) for subtracting the machiningcurrent resistance fall component determined by the integrator (8).

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed description will be presented below for threeembodiments of electrical discharge machining equipment in accordancewith this invention, referring to the drawings described below

FIG. 1 is a schematic drawing of prior art electrical dischargemachining equipment provided with electrical discharge machining gapvoltage sensing means;

FIG. 2 is a schematic drawing of electrical discharge machiningequipment provided with electrical discharge machining gap voltagesensing means in accordance with a first embodiment of this invention;

FIG. 3 is a schematic drawing of electrical discharge machiningequipment provided with electrical discharge machining gap voltagesensing means in accordance with a second embodiment of this invention;and

FIG. 4 is a schematic drawing of electrical discharge machiningequipment provided with electrical discharge machining gap voltagesensing means in accordance with a third embodiment of this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS First Embodiment

Referring to FIG. 2, a machining electrode 1 can be a rod electrode(which is employed with electrical discharge machining equipment forproducing a recess in a workpiece 2), as shown in the drawing, or a wireelectrode (not shown, but employed for wire electrode type electricaldischarge machining). A gap 12, in which electric discharges occur, ismaintained between the machining electrode 1 and the workpiece 2 toallow electric discharges occur therebetween. Means 3 for generating ahigh frequency pulse wave electric voltage is allowed to select variablewave forms. An instrument current transformer 4 or the like is designedto sense the variation rate of electric current. The transformer 4 isactually employed as a means for sensing an inductance fall component.An electric discharge machining gap voltage is sensed at a point 5, atwhich point the circuit of the high frequency pulse wave electricvoltage generating means 3 is connected to the rod machining electrode1, in the case of electrical discharge machining for producing a recessin the workpiece 2. On the other hand, an electric discharge machininggap voltage is sensed by means of an electric feeder means or the like(not shown), in the case of wire electrode type electrical dischargemachining.

A resistor 7 is the resistance fall component sensing means. Thisresistor 7 is essential for the electrical discharge machining equipmentin accordance with the first embodiment of this invention. The electricdischarge machining gap voltage correcting means 6 is inherently anadder. The apparent electric discharge machining gap voltage sensed inthe aforementioned manner is applied to a first element 61 thereof, andan output signal of the electric current variation rate sensing means 4,which represents the inductance fall component, is applied to a secondelement 62 thereof. An output signal of the resistance fall componentsensing means 7 to a third element 63 of the correcting means 6. Boththe inductance fall component and the resistance fall component aresubtracted from the apparent electric discharge machining gap voltage.In this way, the apparent electric discharge machining gap voltage iscorrected to a signal having a value representing the true electricdischarge machining gap voltage.

Adjustment of the second and third elements 62 and 63 is readilypossible by adjusting the output signal of the electric dischargemachining gap voltage correcting means 6 to 0 (zero), under theconditions that the machining gap is short-circuited and a highfrequency pulse voltage is applied between the point 5 and one end ofthe resistor 7.

Second Embodiment

The difference between this embodiment and the first embodiment is thatthe resistor 7, which acts as the resistance fall component sensingmeans in the first embodiment, is replaced by an integrator 8, whichintegrates the output signal of the instrument current transformer 4(the electric current variation rate sensing means which acts as theinductance fall sensing means). This is based on a principle that sinceboth inductance and resistance are originated from a circuit element (awire), and since the resistance of a wire is proportional to theinductance of the wire, integration of an inductance fall becomesequivalent to a resistance fall.

In the same manner as is in the aforementioned embodiment, adjustment ofthe second and third elements 62 and 63 is readily possible by adjustingthe output signal of the electric discharge machining gap voltagecorrecting means 6 to 0 (zero), under the conditions that the machininggap is short-circuited and a high frequency pulse voltage is appliedbetween the point 5 and one end of the instrument current transformer 4.

Third Embodiment

The difference between this embodiment and the first embodiment issimilar to the difference between the first embodiment and the secondembodiment. In the second embodiment, the resistor 7 of FIG. 2, whichacts as the resistance fall component sensing means in the firstembodiment, is replaced by an integrator 8, which integrates an outputsignal of the instrument current transformer 4 (the electric currentvariation rate sensing means which acts as the inductance fall sensingmeans). On the other hand, the difference between this embodiment andthe second embodiment is that an independent second electric currentvariation rate sensing means 9, such as an independent instrumentcurrent transformer, is also provided, in addition to the integrator 8.

The basic principle on which this third embodiment functions is similarto that on which the second embodiment functions. In other words, sinceboth inductance and resistance are originated from a circuit element (awire) and since the resistance of a wire is proportional to theinductance of the wire, integration of an inductance fall is equivalentto a resistance fall. In this third embodiment, however, an independentinstrument current transformer 9 acting as an electric current variatonrate sensing means is particularly effective to improve the accuracy ofthe correction of the electric discharge machining gap voltage.

The adjustment of the second and third elements 62 and 63 is possible inthe same manner as was described above for the first embodiment.

The three embodiments of the aforementioned electrical dischargemachining equipment are provided with electric discharge machining gapvoltage sensing means in accordance with this invention and are providedwith electric discharge machining gap voltage correcting means Thecorrecting means not only subtract the inductance fall component, whichis determined following the electric current variation rate signalsensed by the electric current variation rate sensing means (forexample, an instrument current transformer), from the apparent electricdischarge machining gap voltage sensed across the machining gap, butalso substract the resistance fall component, which is sensed by theresistance fall component sensing means (for example, a resistor).Accordingly, correction is realized not only for the error caused by theinductance fall component, but also for the error caused by theresistance fall component , resultantly enabling the electricaldischarge machining equipment to sense an accurate value of electricdischarge machining gap voltage.

The foregoing description has clarified that this invention hassuccessfully provided electrical discharge machining equipment whichprovides more accurate electric discharge machining gap voltage sensingmeans, which is provided with correcting means for the error caused notonly by the inductance fall component but also by the resistance fallcomponent.

Although this invention has been described with reference to specificembodiments, this description is not meant to be construed in a limitingsense. Various other embodiments and/or modifications of this inventionwill become apparent to persons skilled in the art upon reference to thedescription of this invention. It is therefore contemplated that theclaims will cover any such embodiments and/or modifications as fallwithin the true scope of this invention.

I claim:
 1. An electrical discharge machining apparatus for machining aworkpiece, comprising:a machining electrode; electric discharge gapvoltage sensing means for sensing a gap voltage across a machining gapbetween said machining electrode and the workpiece; high frequency pulsevoltage generating means for supplying high frequency voltage pulsesacross the machining gap; electric current variation rate sensing meansfor determining an inductance fall component by sensing an electriccurrent variation rate across the machining gap; machining currentresistance fall component sensing means for determining a resistancefall component by sensing the intensity of the electric current flowingin the machining gap; and correcting means for correcting the sensed gapvoltage by compensating for the inductance fall component and theresistance fall component.
 2. An electrical discharge machiningapparatus according to claim 1, wherein said machining electrode is awire electrode and said electric discharge gap voltage sensing meanscomprises an electric feeder.
 3. An electrical discharge machiningapparatus according to claim 1, wherein said resistance fall componentsensing means is a resistor.
 4. An electrical discharge machiningapparatus for machining a workpiece, comprising:a machining electrode;electric discharge gap voltage sensing means for sensing a gap voltageacross a machining gap between said machining electrode and theworkpiece; high frequency pulse voltage generating means for supplyinghigh frequency voltage pulses across the machining gap; electric currentvariation rate sensing means for detecting an inductance fall componentby sensing an electric current variation rate across the machining gap;machining current resistance fall component sensing means for sensing aresistance fall component from the electric current variation rate; andcorrecting means for correcting the sensed gap voltage by compensatingfor the inductance fall component and the resistance fall component. 5.An electrical discharge machining apparatus according to claim 4,wherein said machining current resistance fall component sensing meanscomprise integrating means for integrating the electric currentvariation rate to obtain the resistance fall component.
 6. An electricaldischarge machining apparatus for machining a workpiece, comprising:amachining electrode; electric discharge gap voltage sensing means forsensing a gap voltage across a machining gap between said machiningelectrode and the workpiece; high frequency pulse voltage generatingmeans for supplying high frequency voltage pulses across the machininggap; first electric current variation rate sensing means for detectingan inductance fall component by sensing an electric current variationrate across the machining gap; second electric current variation ratesensing means for sensing the electric current variation rate;integrating means for integrating the electric current variation ratesensed by said second electric current variation rate sensing means toobtain a machining current resistance fall component; and correctingmeans for correcting the sensed gap voltage by compensating for theinductance fall component and the resistance fall component.
 7. Anelectrical discharge machining apparatus according to claim 6, whereinsaid first and second electric current variation rate sensing meanscomprise first and second instrument current transformers, respectively.